Control rods are used in nuclear reactors to control the rate of fission of uranium (or other fissile material) and, as such, are capable of absorbing many neutrons without fissioning themselves.
A control rod can be removed from or inserted into the central core of a nuclear reactor in order to control the neutron flux, i.e. to increase or decrease the number of neutrons which will split further uranium atoms. This in turn increases or decreases the thermal power generated by the reactor which (for a light water reactor) determines the amount of steam produced, and thereby the quantity of electricity which is generated.
In a conventional nuclear reactor, control rods are combined into control rod assemblies which are positioned within guide tubes in an individual nuclear fuel element. Such control rod assemblies may typically comprise up to twenty control rods.
Typically, control rod assemblies are arranged vertically within the nuclear reactor core, being inserted into the core from above, with the control rod drive mechanisms being mounted on an upper surface of the reactor pressure vessel. The mass of a control rod assembly will be dependent upon the detailed design of the reactor core but, for a civil nuclear reactor, is typically of the order of up to approximately 100 kg.
In use, one or more of the control rods are partially withdrawn from the reactor core in order to allow a chain reaction to occur. The quantity of control rods which are withdrawn and the distance by which they are withdrawn may be varied to control the reactivity of the reactor.
A typical control rod for a civil nuclear reactor design may be up to approximately 4 m in length. It is therefore necessary that an actuator for a control rod drive mechanism has a travel of at least such a distance, i.e. at least 4 m, in order that the control rod may be fully withdrawn from the reactor core.
Existing civil nuclear reactor control rod drive mechanisms typically use magnetic jack arrangements. A known problem with such arrangements is the inability to use the drive mechanism inside the primary containment boundary of the reactor due to the associated high temperatures and pressures. As a result, the control drive mechanism must be positioned outside the reactor core assembly. This increases the size and complexity of the reactor assembly.